The present invention relates to an electrosurgical apparatus which carries out discission and coagulation on an organic tissue or a surgical target by heat generated by high-frequency power, and, more particularly, to a return electrode separation monitor which discriminates if the return electrode of such an electrosurgical apparatus is properly attached to a patient.
Electrosurgical apparatuses are used in a variety of medical fields, such as surgery and an endoscopic treatment. FIG. 1 is a diagram showing a general structure when an operator 100 performs a treatment using an electrosurgical apparatus. In a treatment, high-frequency power generated by a high-frequency oscillator 106-1 in an electrosurgical apparatus body 106 causes a high-frequency current I to flow across a load of an organic tissue or the like of a patient 103 lying on a bed 105. Based on the Joule heat that is determined by the high-frequency current I and the resistance of the organic tissue, the tissue is vaporized or exploded by a treatment tool 101, thus ensuring discission and coagulation of the tissue.
When the above electrosurgical apparatus is used in mono-polar mode, generally, at least two electrodes are needed. One is an electrode 102 for supplying a high-frequency current to a patient (hereinafter this electrode will be called "active electrode"). The active electrode 102 in use often has a sharp shape like a needle or a blade in order to narrow the area of electrode-tissue contact and improve the current density at a surgical area.
The other electrode is a so-called return electrode (split counter electrode plate) 104 that collects the high-frequency current, which flows through a patient from the active electrode 102, and returns the high-frequency current to the electrosurgical apparatus body 106. As this return electrode 104 also serves to prevent a burn at a portion of contact to a tissue, the return electrode 104, unlike the active electrode 102, is shaped to have a wide area to reduce the current density at the contact. If the return electrode 104 is not in proper contact to a patient, local current concentration may cause a burn.
Recently therefore has been proposed a return electrode separation monitor and method in which the return electrode 104 is constructed by two conductors and which previously discriminate if the return electrode 104 is properly attached to a patient by detecting an AC-like impedance between the two conductors.
One conventional return electrode separation detecting method detects an AC-like contact impedance using the property of a transformer such that the impedances appearing on the primary winding and the secondary winding is determined by the winding ratio of the transformer, and compares a detection signal according to the contact impedance with a given threshold value for abnormality determination to determine an abnormality.
According to this return electrode separation detecting method, however, the characteristic of an AC impedance detecting section is determined by the characteristics of the transformer so that a signal corresponding to the contact impedance to be detected varies due to a variation in various characteristics of the transformer. Further, the contact impedance between the return electrode and a patient is actually as low as several tens of ohms and the contact impedance when the return electrode is gradually separated varies in the order of several ohms. This makes it practically very difficult to detect the contact impedance by the transformer alone, not to mention the large variation of the contact impedance.
If as idealistic a transformer as possible is used, a contact impedance which varies very slightly may be detected reliably. Practically, however, it is very hard to manufacture such an idealistic transformer, which, if manufactured, would lead to an increased cost.
When abnormality determination is carried out with a predetermined, specific fixed threshold value, the area of separation of the return electrode up to the point where an abnormality is determined differs depending on subject persons because the contact impedance significantly varies person by person. In an extreme case, even if the return electrode is fully in contact with a person who has a high contact impedance, an alarm may be issued, hindering a procedure. For a person who has a low contact impedance, on the other hand, the area of separation of the return electrode up to the point where an abnormality is determined becomes large, resulting in a lower detection accuracy which makes it more likely to cause a burn.
In short, the prior art has a difficulty in detecting the contact impedance which varies in the order of several ohms, has a large variation in the detected signal itself, cannot enhance the detection precision, and suffers a significant person-dependent variation in the area of separation of the return electrode up to the point where an abnormality is determined because abnormality determination is performed on the contact impedance, which considerably differs from one person to another, based on a predetermined, specific threshold value (fixed). Depending on a patient, it may not be determined as abnormal even when the area of separation becomes large.